(1) Field of the Invention
The present invention generally relates to a method of removing sulfur oxides such as sulfur trioxide (SO3) contained in flue gas from a coal or oil-fired boiler. In particular, the present invention is directed to reducing the level of SO3 contained in flue gas by injecting an atomized chloride salt solution or dry chloride particulate matter continuously into the flue gas at a point downstream or upstream of an air pre-heater to absorb the SO3.
(2) Description of the Related Art
Flue gases from coal and oil-fired boilers contain acid gases like sulfur dioxide (SO2) and SO3. SO2 is gaseous under all practical conditions occurring in a plant and can be controlled by a number of different gas absorption technologies. SO3 prevails in gaseous state in the gases leaving the heat recovery sections of a boiler at levels between 1 and 100 ppm by volume.
One known gas absorption technology is wet flue gas desulfurization (WFGD). In WFGD, the flue gas enters a large vessel, e.g., a spray tower or absorber, which is generally referred to as a wet scrubber, where it is sprayed with water slurry, i.e., a mixture of water and at least partially insoluble matter, e.g., lime, limestone, or the like. The calcium in the slurry reacts with the SO2 to form calcium sulfite or calcium sulfate. A portion of the slurry from the reaction tank is pumped into the thickener, where the solids settle before going to a filter for final dewatering. The calcium sulfite waste product is usually mixed with fly ash and fixative lime and disposed of in landfills. Alternatively, gypsum can be produced from the WFGD waste product or the WFGD waste product may be recycled.
In actual flue gases with an appreciable content of water vapor, the SO3(gas) molecules are probably associated with water molecules when the flue gas temperature has dropped down in the back-end of the boiler and exist as H2SO4(gas). On cooling of a flue gas, there is a temperature where sulfuric acid is formed from the gaseous state. This is the sulfuric acid dew point, which is a function of the partial pressures of water and SO3.
Precipitated SO3 has various undesirable effects. For example, heat recovery equipment must be designed to withstand the rapid corrosion caused by precipitating on cold metallic surfaces. Also, if precipitated in a flue gas plume, it contributes to opacity and mass loading in the flue gas plume emitted.
Presently, one method to reduce the undesirable effects is to inject solid alkaline particulate matter in the flue gas, such as calcium carbonate or magnesium oxide. One disadvantage of this method is that a large excess of absorbent is needed to achieve high absorption unless the admixture of the solid alkaline material is uniform throughout the flue gases. Another disadvantage of this method is that alkali consumption is high since the SO3 absorption relies on comparatively slow solid-state reactions. A substantial amount of solid end product is produced using this method due to the poor utilization of the material, rendering it more difficult to collect in an electrostatic precipitator, or less suitable for reutilization.
Another method to reduce undesirable effects of precipitated SO3 is to dry the sodium bisulfite solution by the heat of the flue gas and simultaneously, SO3 is absorbed, sodium sulfate is produced, and SO2 is released. The present methods generally fail to alleviate such issues as the high cost of sodium bisulfite and sparing availability of the material. Also, the solubility of sodium bisulfite is comparatively low; leading to that in order to inject a given amount of active material, a comparatively large amount of “ballast” water has to be injected.